1. Field of the Invention
This invention relates to oscillators in general and more particularly to a voltage controlled oscillator that can rapidly change frequencies over a wide range.
2. Description of Related Art
Voltage controlled oscillators are used in many electronic applications such as telecommunications. A voltage-controlled oscillator (VCO) is a circuit that generates an oscillating signal at a frequency proportional to an externally applied control voltage. Modern electronics often require a VCO to operate over large frequency ranges. Some applications require the voltage controlled oscillator to rapidly change the carrier frequency. These types of oscillators are referred to as agile voltage controlled oscillators.
The ability of a voltage controlled oscillator to change frequencies can be measured by 3 dB modulation bandwidth and tuning time. Normally, the 3 dB modulation bandwidth is determined at a low modulation index. This means that the deviation of the carrier frequency is significantly less than the carrier frequency itself or the amplitude of the modulating signal is small.
There are applications where the carrier frequency must change significantly in a short period of time. In these applications, the frequency deviation is large and the amplitude of the modulating signal applied to the tuning port has a large value. In some cases, the amplitude may cover the entire tuning voltage range. Tuning the frequency fast requires the modulating signal applied to the tuning port to have a large rise time or dv/dt, which is measured in volts per second.
For example, the modulating signal applied to the tuning port of a VCO could increase from 1 volt to 11 volts in 2 nanoseconds. This gives a rise time of 5 volts per nanosecond. This sharp or large rise time signal applied to the tuning port penetrates beyond the tuning components (typically varactor diodes) and can completely disrupt the normal operation of the VCO. Voltage controlled oscillators that are designed with a wide 3 dB modulation bandwidth such as 10 MHz cannot handle a 5 volt per nanosecond tuning signal.
Referring to FIG. 1, a schematic diagram of a typical Clapp voltage controlled oscillator 20 is shown. The typical frequency tuning range of the Clapp oscillator is approximately a 2:1 ratio. For a wide frequency tuning range capacitors C1 and C2 should be large. When a large rise time signal is applied to the tuning port Vtune, capacitor C1 will start to charge. The charge time is determined by the capacitance value of C1 and the total resistance. The total resistance is equal to Rtune+R4+R2, where R tune is the tuning port source impedance and R4, R2 are the resistances of resistors R4 and R2.
Referring to FIG. 2, a graph of the frequency response of the Clapp oscillator of FIG. 1 when a sharp step-up voltage is applied to the tuning port is shown. If capacitor C1 has a value of 1000 picofarads, R4 is 4000 ohms and R2 is 4000 ohms, the time constant will be 8 microseconds. The 3 dB modulation bandwidth is 60 KHz. The fast rise time of this signal appears almost without attenuation at the base of transistor Q1. The fast signal causes the DC operation point of transistor Q1 to move, stopping or interrupting oscillation. The oscillation resumes as capacitor C1 charges and the transistor base voltage comes back to a normal steady state value.
FIG. 3 shows a graph of the frequency response of the Clapp oscillator of FIG. 1 when a sharp step-down or decreasing voltage is applied to the tuning port. The effect is similar for a decreasing voltage. If the modulating signal applied to the tuning port of a VCO decreases from 11 volts to 1 volt in 2 nanoseconds, the negative pulse on the base of transistor Q1 results in the transistor turning off, stopping oscillation. The transistor may even enter an operating condition where it can sustain damage during the positive or negative pulse.
Turning to FIG. 4, a schematic diagram of a typical Colpitts voltage controlled oscillator 30 is shown. The Colpitts oscillator handles a narrower frequency range than the Clapp oscillator circuit. Also, the 3 dB modulation bandwidth and tuning speed are better in a Colpitts oscillator than a Clapp oscillator. The frequency limitations for the Colpitts oscillator are capacitors C1 and C2. In the Colpitts oscillator, capacitor C1 has a lower value and charges faster. The frequency tuning range of a Colpitts oscillator is a 5 to 10 percent change in frequency. A sharp step up voltage applied to the tuning port of the Colpitts oscillator has less effect because the resonator inductor L1 is a low shunt impedance to ground and capacitor C1 has a high series impedance to the base of transistor Q1. Colpitts oscillators typically do not have a problem with oscillation interruption during fast tuning. Unfortunately, they are limited to being able to tune frequencies in only a narrow frequency range.
While various oscillators have been used, a continuing need exists for a voltage controlled oscillator that has improved electrical performance. In particular, a voltage controlled oscillator that can rapidly change frequencies over a wide frequency range without an interruption in oscillation is needed.